Recently, backlights for liquid crystal display are becoming larger and cold-cathode fluorescent lamps to be used for backlights are becoming longer.
Accordingly, the discharge voltage is becoming higher. So is the discharge impedance.
The EEFL requires a higher discharge voltage.
Because a large surface light source for a liquid crystal display television or the like requires that the brightness of the surface light source should be uniform, the surface light source is provided for each cold-cathode fluorescent lamp with a mechanism which detects the currents that flows in the cold-cathode fluorescent lamp and feeds the detection result to a control circuit to keep the lamp current constant, as shown in FIG. 12.
Many of the conventional discharge lamp lighting systems generally light discharge lamps by setting the electrode on one side of a cold-cathode fluorescent lamp to a high voltage and driving the electrode at the other end with the GND (ground) level. Such a lighting scheme is called “single-side high voltage driving”, and the drive method is advantageous in that the lamp current control is easy so that a lighting circuit is easy to configure.
As cold-cathode fluorescent lamps become longer, the discharge voltage of the cold-cathode fluorescent lamps gets higher and the impedance of discharge lamps gets higher, so that the difference in brightness between the high-voltage side and low-voltage side of the cold-cathode fluorescent lamp stands out. Such a phenomenon is called “nonuniform brightness”.
While the nonuniform brightness phenomenon does not distinctly occur on a cold-cathode fluorescent lamp alone, it apparently occurs when the cold-cathode fluorescent lamp is placed closer to a proximity conductor, such as a reflector. (See Japanese Laid-Open Patent Publication (Kokai) No. H11-8087 and Japanese Laid-Open Patent Publication (Kokai) No. H11-27955.)
As single-side high voltage driving results in large nonuniform brightness, a so-called double-side high voltage driving system or a floating system is proposed to reduce nonuniform brightness by driving both ends of a cold-cathode fluorescent lamp with high voltages of opposite phases, as shown in FIG. 13. Because the voltage to be applied to each electrode of a cold-cathode fluorescent lamp becomes a half, this system is advantageous in driving an elongated cold-cathode fluorescent lamp or external electrode fluorescent lamp which require a high voltage.
As the voltage to be applied to each electrode becomes a half, a leak current which is the flow of the current due to a parasitic capacitance produced around a discharge lamp becomes smaller, making the brightness of the cold-cathode fluorescent lamp more uniform.
In addition, the voltage to be applied to the windings of a step-up transformer becomes lower, increasing the safety of the step-up transformer.
It is said that double-side high voltage driving is suitable for driving elongated cold-cathode fluorescent lamps in a large surface light source.
As a cold-cathode fluorescent lamp is driven with a high voltage, however, there is large static noise generated from the cold-cathode fluorescent lamp.
As the static noise affects the liquid crystal display, every other cold-cathode fluorescent lamps are alternately driven with outputs different in phase by 180 degrees to cancel out static noise generated from the cold-cathode fluorescent lamp, as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2000-352718.
FIG. 15 shows one example of the structure in which the secondary winding of a transformer takes a floating structure to provide outputs of opposite phases, which are connected to one ends of cold-cathode fluorescent lamps whose other ends are connected together so that the cold-cathode fluorescent lamps are driven in the form of parallel connection.
The lamp currents of individual fluorescent lamps are detected by current detection means CDT1 to CDT4 respectively, are feedback to voltage sources WS1 to WS4 to make the lamp currents uniform and stable.
As adjoining cold-cathode fluorescent lamps are driven with voltages different in phase by 180 degrees, therefore, static noise generated from the cold-cathode fluorescent lamp is canceled, thus reducing the influence on the liquid crystal display.
FIG. 16 shows one example in which the above method is further modified. A transformer with a floating structure is provided for each cold-cathode fluorescent lamp, and every other cold-cathode fluorescent lamps are alternately driven with outputs different in phase by 180 degrees to cancel out static noise.
Further, as the wires of a high voltage are long according to the method illustrated in FIG. 16, the structure that is shown in FIG. 17 is taken where a leakage flux transformer is arranged on either side to make the high-voltage wires shorter.
While each of FIGS. 16 and 17 exemplarily shows an AC power source, in an inverter circuit for an actual large surface light source is provided with a lamp current control circuit as shown in FIG. 12 for each transformer. This makes the scale of the circuit huge.
A problem that the circuit scale of an inverter circuit in a large surface light source system becomes huge can be overcome by means of driving multiple cold-cathode fluorescent lamps used in a surface light source in parallel to thereby make the lamp currents of the individual discharge lamps uniform. The solution is proposed by the inventors of the present invention in U.S. Laid-Open Patent Publication No. 2004-0155596-A1 (corresponding to Japanese Laid-Open Patent Publication (Kokai) No. 2004-00374) and illustrated in FIG. 18.
According to the single-side high voltage driving system, one electrode side of a cold-cathode fluorescent lamp becomes a high voltage while the other electrode side is the GND (ground) level. When multiple cold-cathode fluorescent lamps are driven in parallel by the method illustrated in FIG. 18 and proposed in U.S. Laid-Open Patent Publication No. 2004-0155596-A1, electrodes on one side of adjoining ones of multiple cold-cathode fluorescent lamps are in phase.
Such a single-side high voltage driving system has a problem of large nonuniform brightness. In addition, static noise generated from the cold-cathode fluorescent lamp is large, which may influence the liquid crystal display.
To cut off static noise generated from a surface light source, therefore, it is necessary to insert a conductive film coated with ITO (Indium Trioxide) or so between the surface light source and the liquid crystal display panel.
Such nonuniform brightness occurs when a cold-cathode fluorescent lamp is placed close to a reflector and is such that the high-voltage side is bright while the low-voltage side is dark. It is said that such nonuniform brightness is not avoidable in a large surface light source.
The nonuniform brightness increases when the impedance of a cold-cathode fluorescent lamp is high or when the parasitic capacitance around the cold-cathode fluorescent lamp is large because the current flows to a nearby conductor via the parasitic capacitor. Even when the drive frequency of a cold-cathode fluorescent lamp becomes higher, therefore, nonuniform brightness becomes greater.
It is often the case where the lamp current is made smaller to extend the service life of a cold-cathode fluorescent lamp for a backlight for a liquid crystal display television. Reducing the lamp current also means an increase in the impedance of the cold-cathode fluorescent lamp.
As an elongated cold-cathode fluorescent lamp is used in a large liquid crystal display television and originally has a high impedance, the impedance of the cold-cathode fluorescent lamp becomes higher for the two reasons mentioned above, so that particularly, nonuniform brightness is likely to occur.
If a cold-cathode fluorescent lamp is long, the outside diameter should be made larger to provide a strength. While a cold-cathode fluorescent lamp for a backlight (surface light source) for a notebook type personal computer is normally 1.8 mm to 2.7 mm in diameter, a cold-cathode fluorescent lamp in use for a backlight (surface light source) for a liquid crystal display television is about 3 mm to 5 mm in diameter. The increased outside diameter of a cold-cathode fluorescent lamp means that the parasitic capacitance produced between the cold-cathode fluorescent lamp and the reflector becomes greater.
In a large surface light source, therefore, not only the impedance of the cold-cathode fluorescent lamp is high but also the parasitic capacitance is high, resulting in overlapped conditions of making nonuniform brightness likely to occur. In view of this, it is said to be difficult to drive a large liquid crystal display backlight having an elongated cold-cathode fluorescent lamp on a high frequency.
Because the nonuniform brightness phenomenon is such that a high-potential portion near the electrode of a cold-cathode fluorescent lamp becomes bright while a low-potential portion becomes dark, nonuniform brightness occurs less in the double-side high voltage driving system than in the single-side high voltage driving system. (See Japanese Laid-Open Patent Publication (Kokai) No. H11-8087 and Japanese Laid-Open Patent Publication (Kokai) No. H11-27955.)
In the case of double-side high voltage driving, portions near the electrodes on both sides become bright while the center portion becomes dark. Nonuniform brightness in this case is considerably smaller than nonuniform brightness in the case of single-side high voltage driving. When double-side high voltage driving is employed, therefore, the drive frequency can be increased.
With double-side high voltage driving, an inverter circuit requires two outputs of opposite phases.
In the case of the structure where the outputs of the inverter circuit are provided with leakage flux transformers and are connected directly to electrodes on both sides of a cold-cathode fluorescent lamp, the inverter circuit provides two outputs different in phase by 180 degrees. In this case, however, the two outputs of opposite phases of the inverter circuit should not necessarily become uniform.
With nonuniform outputs, the voltage applied to the electrode on one side of the cold-cathode fluorescent lamp becomes greater, while the voltage applied to the electrode on the other side of the cold-cathode fluorescent lamp becomes lower, making the loads on the outputs of the inverter circuit uneven. Such biasing of outputs is likely to occur when the power factor as seen from the primary side of the step-up transformer is improved and the copper loss is reduced by using the leakage flux transformer in the step-up transformer and causing resonation of the leakage inductance of the leakage flux transformer and the capacitive component of the secondary circuit.
The technique of achieving high efficiency of an inverter circuit using the resonance technique is disclosed in U.S. Pat. No. 5,495,405 by one of the inventors of the present invention. That is, biasing of outputs is hard to occur in a conventional inverter circuit which uses a non-leakage flux transformer having a low leakage inductance as the step-up transformer at the output stage and uses a ballast capacitor to stabilize the lamp current. The biasing of outputs is a particular phenomenon which occurs when a scheme of acquiring a high efficiency is performed by working out the invention in U.S. Pat. No. 5,495,405.
When an inverter circuit has two outputs whose output voltages differ in phase from each other by 180 degrees, a resonance circuit is constructed for each of the outputs of opposite phases as shown in FIG. 13. When the two resonance circuits are constructed not in association with each other, the resonance frequencies of the resonance circuits should not necessarily match with each other.
If the resonance frequencies of the resonance circuits do not match with each other, as shown in FIG. 14, the step-up ratios of the outputs of the inverter circuit differ even when the resonance circuits are driven with the same frequency, thus making the voltages to be applied to the electrodes of the cold-cathode fluorescent lamp different from each other. As a result, the outputs of the inverter circuit are unbalanced.
The unbalance is originated from the difference in the resonance frequencies of the outputs of opposite phases caused by the difference in leakage inductances of the leakage flux transformers to be used at the outputs of the inverter circuit or the difference in capacitive components of the secondary circuit.
In an actual surface light source system, a current distributor module is connected to each electrode of the cold-cathode fluorescent lamp or the size precisions of the cold-cathode fluorescent lamp and the reflector which includes the effect as a proximity conductor vary, thus causing considerable unbalance of parasitic capacitances.
There are fluctuations in leakage inductances of the leakage flux transformers, which are the cause of making the resonance frequencies of the resonance circuits unmatched with each other.
When the resonance frequencies do not match with each other, the outputs become unbalance so that the electrodes on both sides of the cold-cathode fluorescent lamp cannot be driven uniformly. As a result, excessive power concentration occurs on one output, leading to nonuniform heat generation of the inverter circuit.
To prevent the biasing of outputs, the resonance frequencies of the resonance circuits for the outputs of opposite phases should be made uniform.
The following will discuss the problem of the prior art from viewpoint of static noise.
To reduce static noise, it is effective to cancel static noise by driving adjoining cold-cathode fluorescent lamps with outputs of opposite phases. FIGS. 15 to 17 show examples of the structure. To drive cold-cathode fluorescent lamps in the mentioned manner, a single transformer having outputs of opposite phases is provided for every set of two cold-cathode fluorescent lamps which are driven in opposite phases.
In the example shown in FIG. 15, however, the electrodes on one side of adjoining cold-cathode fluorescent lamp become high potentials of opposite phases while the other electrodes are at the GND (ground) potential. In this case, the presence of the leak current flowing via a parasitic capacitor Csm produced between the adjoining cold-cathode fluorescent lamps on the high-voltage side makes nonuniform brightness worse than the single-side high voltage driving system in the case shown in FIG. 18. This undesirably requires that the backlight with such a structure should be driven with a relatively low frequency.
One solution to this problem is to realize double-side high voltage driving by driving a single cold-cathode fluorescent lamp with a single transformer as shown in FIG. 16.
Because multiple high-voltage lines run across in the casing of the surface light source according to the method, however, the parasitic capacitance becomes unbalanced.
In addition, the individual cold-cathode fluorescent lamps are alternately driven in opposite phases, thus requiring more transformers than the structure shown in FIG. 15.
The structure shown in FIG. 17 has a greater number of transformers to prevent high-voltage crossover lines so that the transformers are arranged on both sides of the cold-cathode fluorescent lamps to achieve double-side high voltage driving, and changes the phase of the drive voltage for every other cold-cathode fluorescent lamp to reduce static noise. The structure apparently needs a significant number of transformers and control circuits.
Although a switching circuit and a control circuit are not shown in FIGS. 16 and 17, the actual inverter circuit system for a liquid crystal display television has additional circuits of detecting the lamp currents of the individual cold-cathode fluorescent lamps and controlling the respective cold-cathode fluorescent lamps, the inverter circuit has a very large scale.
None of the circuits shown in FIGS. 15 to 17 do not solve the problem of the outputs being unbalanced due to the deviation of the resonance frequency of the secondary circuit.
In view of the above, there has been demands for a low-cost surface light source system and an inverter circuit for multiple lamps, which reduces nonuniform brightness and static noise, and fulfills the requirement that lamp currents of individual cold-cathode fluorescent lamps should be uniform and stabilized.